1. Field of the Invention
This invention is directed to valves and to a float collar flow valve for use in wellbore operations. This invention is particularly directed to a float collar valve for use in running a casing string into a well and in cementing such a string; to a locking mechanism for preventing valve stem movement; and to a valve guide formed so as to be able to receive parts of a new locking mechanism.
2. Description of the Prior Art
In running a casing string in a bore hole, it is necessary to provide a valve for controlling the rate of flow of fluid from the bore hole into the casing. If the fluid is not controlled as it enters the bottom of the casing, the fluid pressure in the well may be sufficient to cause the fluid to blow out through the top of the casing. On the other hand, it is necessary to fill the casing gradually as it is lowered to compensate for the head of fluid on the outside of the casing to prevent implosion or collapse of the casing. Various self-fill float valves have been utilized for limiting the rate of flow of fluid into the casing, as it is being lowered into the bore hole.
After the casing has reached the desired depth, it is cemented in place by conducting cement down the casing and out through the bottom of the casing, where it flows upwardly through the annular space between the casing and the bore hole wall. Accordingly, it is necessary to provide a valve which will resist the back pressure of the cement slurry in the annulus. The back pressure valve, however, should prevent flow of the slurry back into the casing, while the self-fill float valve allows a restricted flow into the casing. Since these valves operate under different conditions and during different portions of the casing cementing process, it has been proposed to combine these valves in a single unit and to render the valves operative by various means. It is important that the valves operate effectively and reliably to ensure a successful cementing operation.
The flow control float collar valve described in U.S. Pat. No. 3,385,372 was an attempt to solve the problems already described. In this valve there is a collar having a tubular body with a valve guide in the body. A valve seat is formed in the body above the valve guide. A movable valve element is mounted in the valve guide in position to move upwardly into engagement with the valve seat and is spring biased toward the valve seat. A fluid reaction surface is mounted on the valve stem below the valve element. The fluid reaction surface is biased downwardly by a spring that applies a greater axial force to the valve stem than does the spring biasing the valve element. The fluid reaction surface is arranged to exert an upwardly directed force on the valve stem in response to the flow of fluid upwardly through the casing. Thus, the reaction surface restricts or limits the rate of flow upwardly by displacing the first valve element against the valve seat. There is a shear pin joining the reaction surface to the valve stem and, upon pumping fluid down the casing at a predetermined rate, the force of the fluid on the reaction surface breaks the shear pin and causes the reaction surface to be displaced downward and thereby rendered ineffective. After the shear pin is broken, the valve element is released and its spring urges the valve element against the valve seat. Therefore, the collar is converted to a back pressure valve.
The valve has numerous disadvantages. The shear pin can be sheared by any force of sufficient strength such as the force of vibration when the valve is shipped or moved or the force of any impact on the valve e.g. if it is dropped on the rig floor. Once the shear pin is broken, the valve cannot converted back to a self-fill flow valve.
Another disadvantage of the valve is the limited flow rates of wellbore fluids or cement which are possible through the valve due to the blocking of the flow path by the surface reactive areas necessitated by the valve design. Because of this limitation on flow rate, the interior of the casing must be filled with wellbore fluid by pumping the fluid down the casing, rather than letting fluid enter the casing at the bottom of the casing string. Fluid simply cannot enter at the bottom of the casing fast enough to provide adequate pressure equalization between casing and wellbore due to the flow restrictions of the valve. Therefore fluid must be pumped down the casing to prevent casing collapse. Also, if the flow from the wellbore into the bottom of the casing string was at a relatively high rate, the shear pin could be sheared closing the valve and preventing any fluid from entering the casing; again causing casing collapse.
A float collar shell is a cylindrical shell member used to contain the valve. In use it forms a part of the casing string. The dimensions of a standard float collar shell are such that a valve such as the valve of U.S. Pat. No. 3,385,372 does not fit in the standard shell because of the added length necessary to accommodate the valve's fluid reactive mechanism. Also, the prior art flow valves are manufactured and delivered as a unit which cannot be modified, i.e. they cannot be converted on the rig to an automatic fill mode because the fluid reactive mechanism is an integral part of the valve and must be assembled with the valve during manufacture. Also it is practically impossible to add a shear pin to an already manufactured valve such as the shear pin necessary in the valve of U.S. Pat. No. 3,385,372.
One of the primary disadvantages of a valve according to U.S. Pat. No. 3,385,372 is the imprecision of and hence the unpredictability of the performance of the shearing mechanism and the valve closure mechanism. This imprecision is due to the numerous engineering, design, and manufacturing characteristics of these mechanisms. The shearing of the shear pin depends on all of the following variables:
a. The pressure differential across the fluid reactive member. PA1 b. The material and dimensions of the pin. PA1 c. The characteristics of the spring which biases the fluid reaction surface downwardly (thereby creating an upward, i.e. shearing, force on the pin).
A failure or inadequacy related to the design, selection, or manufacture of any of these variables can result in a shear pin that shears too soon, too late, or which never shears at all.
In accordance with .sctn. 1.56 of the 37 C.F.R., applicants are aware of U.S. Pat. No. 3,385,372 and a publication entitled "Tests find hammering, fluid cutting, erosion cause float shoe failures," by William D. Stringfellow published Jan. 21, 1985 by OIL & GAS JOURNAL, copies of which are submitted herewith.